Apparatus and methods for controlling operation of a single-phase voltage regulator in a three-phase power system

ABSTRACT

Provided are methods for controlling operation of a voltage regulator of a single-phase of a three-phase power system to regulate a measured voltage. One of the methods includes recording a first elapsed time between detecting a first excursion of the measured voltage from an in-band area to an out-of-band area, and a first return of the measured voltage to the in-band area. The method also includes recording a second elapsed time period (dip period) between detecting the first return and a second excursion of the measured voltage from the in-band area to an out-of-band area. If the second elapsed time period is less than a predetermined dip time period, causing a tap position change of the voltage regulator upon expiration of a countdown period initiated upon detecting the first excursion, thereby adjusting the measured voltage to the in-band area while allowing a voltage drop of limited length.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to another patent applicationentitled “An Apparatus and Methods for Providing a Voltage Reduction forSingle Phase Voltage Regulator Operation in a Three-Phase Power System”,filed on Oct. 21, 2005, naming Casper A. Labuschagne as inventor. Thisapplication claims benefit under 35 U.S.C. §119(e) of U.S. ProvisionalApplication entitled “An Apparatus and Methods for Controlling Operationof a Single-Phase Voltage Regulator Operation in a Three-Phase PowerSystem”, filed on Oct. 21, 2005, naming Casper A. Labuschagne asinventor, the complete disclosure thereof being incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention generally relates to power system control, andmore specifically, to an apparatus and method for controlling operationof a single-phase voltage regulator in a three-phase power system.

Electric utility systems or power systems are designed to generate,transmit and distribute electrical energy to loads. In order toaccomplish this, power systems generally include a variety of powersystem elements such as electrical generators, electrical motors, powertransformers, power transmission lines, distribution lines, buses andcapacitors, to name a few. As a result, power systems must also includea number of regulators having associated control devices, and manyprotective devices having associated protective schemes to protect thepower system elements from abnormal conditions such as electrical shortcircuits, overloads, frequency-excursions, voltage fluctuations, and thelike.

In general, protective devices and their associated protective schemesact to isolate a power system element(s) (e.g., a generator,transformers, buses, motors, etc.) from the remainder of the powersystem upon detection of the abnormal condition or a fault in, orrelated to, the power system element(s). Such protective devices mayinclude different types of protective relays, surge protectors, arc gapsand associated circuit breakers and reclosures.

Regulators and their associated control devices are utilized to regulatethe voltage level in the power system. For example, a number ofsingle-phase step voltage regulators may be coupled to the varioustransmission, sub-transmission and distribution lines (collectively,“distribution lines”) to enable voltage regulation of the distributionline to, for example 13 kV±10 percent, during a wide range of loadconditions (e.g., a plant coming on-line). Such voltage regulators areoften placed adjacent to a step-down power transformer and generallyinclude an autotransformer having a single winding (e.g., a serieswinding), which is tapped at some tap position along the winding toprovide a desired voltage flow.

A typical step voltage regulator may have a 100 percent exciting windingin shunt with the distribution line on the source side, and operate tomaintain a voltage on the load side of the distribution line. Thevoltage is maintained within a desired voltage bandwidth by means of a10 percent tapped buck/boost winding connected in series with thedistribution line. The series winding has taps connected to stationarycontacts of a tap changer dial switch, where the tap changer dial switchincludes a pair of rotatable selector contacts driven by a reversiblemotor into sequential engagement with the pairs of contacts. Forexample, the tap changer dial switch may enable an ability to change theeffective turns ratio from input to output±10 percent in 32 steps of ⅝percent each or 0.7 V. A voltage control device monitors thedistribution line voltage and current and determines the proper tapposition based on the measured distribution line voltage.

Voltage regulators operate via a comparison of an actual measuredvoltage (i.e., a secondary distribution line voltage) to some internalfixed reference voltage, or center-band voltage. Any voltage differenceis amplified and used to control operation of the voltage regulator viathe voltage control device. Thus, if the measured voltage is too high orin an out-of-band (OOB) area above a center-band area or center-bandvoltage range, the voltage regulator is commanded by the voltage controldevice to execute a tap position change to produce a lower voltage, andif the voltage is too low, or in an OOB area below the center-band area,the voltage regulator is commanded by the voltage control device toexecute a tap position change to produce a higher voltage.

Because currents resulting from a fault can easily exceed 10,000 amperes(amps) and because the voltage control device is designed to utilizecurrents and voltages much less than those of the distribution lines,the currents and voltages are stepped-down via current and voltagetransformers, respectively. As is known, the three-phase current andvoltages are commonly referred to as the primary current and voltages,while the stepped-down current and voltages are referred to as thesecondary current and voltages, respectively. The stepped-down secondarycurrent and voltages are digitized and then utilized by amicrocontroller of the voltage control device to determine correspondingphasors representative of the primary current and voltages. The phasorsare then used by the microcontroller while executing the voltage controllogic scheme of the voltage control device to determine whether a tapposition change is required by the voltage regulator (discussed below).

One voltage control scheme commonly referred to as a definite timecharacteristic, includes setting a countdown timer, referred to hereinas a First timer, upon detection of a measured voltage in an OOB area.Such a voltage excursion into the OOB area is determined by comparing avoltage phasor, calculated from secondary voltages provided by thevoltage transformer, to the center-band area. If the measured voltageremains in the OOB area during a countdown time period of the Firsttimer, a tap position change is initiated to either lower the loadvoltage (due to a high OOB voltage) or raise the load voltage (due to alow OOB voltage). If the measured voltage does not remain in the OOBarea for the countdown time period, and instead the measured voltagedips in-band momentarily or otherwise, the First timer resets to itscountdown time period. The First timer will again begin its countdowntime period upon detection of a second voltage excursion into the OOBarea. As a result, the elapsed time period of the first voltageexcursion into the OOB area is ignored. If the voltage again dipsin-band, the First timer, executing the countdown for second time, againresets to it countdown time period. Thus, for cases where the measuredvoltage is oscillating around the in-band/OOB edge (i.e., dipping in andout of the in-band area), the voltage control device may not issue aneeded tap position change command to the voltage regulator due torepeated First timer resets. Accordingly, the feedline voltage is notoptimized to the in-band area.

SUMMARY OF THE INVENTION

In accordance with the invention, disclosed are an apparatus and methodsto enable improved voltage regulator control, especially for those caseswhere an occasional momentary load drop occurs or where the measuredvoltage of the feedline, or distribution line, oscillates around thein-band/OOB edge between one of the OOB areas and the in-band area.

In accordance with an aspect of the invention, an apparatus and methodare provided for controlling operation of a voltage regulator via a tapposition change where the voltage regulator is operatively coupled to asingle-phase of a three-phase power system to regulate a voltage of thesingle-phase to an in-band area for delivery to a load. The apparatusincludes a means for deriving a digitized voltage sample streamrepresentative of a time-varying measured voltage of the single-phase,and a microcontroller operatively coupled to the means for deriving thedigitized voltage sample stream. The microcontroller includes, amongother things, a microprocessor and a memory operatively coupled to themicroprocessor. The microcontroller is programmed to (1) start acountdown time period of a first timer upon detecting a first excursionof the measured voltage from the in-band area to an out-of-band area, orOOB area, (2) upon detecting a first return of the measured voltage tothe in-band area, record a first elapsed time of the first timer that isbased on a time elapsed between the first excursion and the firstreturn, reset the first timer to the countdown time period, and start asecond timer, and (3) upon detecting a second excursion of the measuredvoltage from the in-band area to the out-of-band area, record a secondelapsed time of the second timer that is based on a time elapsed betweenthe first return and the second excursion, and compare the secondelapsed time to a predetermined dip time period. If the second elapsedtime is less than the predetermined dip time period, the microcontrolleris programmed to start an adjusted countdown time period of the firsttimer, and if the second elapsed time is more than the predetermined diptime period, the microcontroller is programmed to start the countdowntime period of the first timer upon subsequent entry of the measuredvoltage into the OOB area. The adjusted countdown time period is equalto the countdown time period minus a sum of the first and second elapsedtimes.

In accordance with another aspect of the invention, an apparatus andmethod are provided for controlling operation of a voltage regulator viaa tap position change where the voltage regulator is operatively coupledto a single-phase of a three-phase power system to regulate a voltage ofthe single-phase to an in-band area for delivery to a load. Theapparatus includes a means for deriving a digitized voltage samplestream representative of a time-varying measured voltage of thesingle-phase, and a microcontroller operatively coupled to the means forderiving the digitized voltage sample stream. The microcontrollerincludes, among other things, a microprocessor and a memory operativelycoupled to the microprocessor. The microcontroller is programmed to (1)start a countdown time period of a first timer upon detecting a firstexcursion of the measured voltage from the in-band area to anout-of-band area, (2) upon detecting a first return of the measuredvoltage to the in-band area, start a second timer, and (3) upondetecting a second excursion of the measured voltage from the in-bandarea to the out-of-band area, record a dip time of the second timerbased on a time elapsed between the first return and the secondexcursion, and compare the dip time to a predetermined dip time period.If the dip time is less than the predetermined dip time period and ifthere is not a second return of the measured voltage to the in-bandarea, the microcontroller is programmed to cause the tap position changeof the voltage regulator upon expiration of the countdown time period.The tap position change adjusts the measured voltage from theout-of-band area to the in-band area.

In accordance with yet another aspect of the invention, an apparatus andmethod are provided for controlling operation of a voltage regulator viaa tap position change where the voltage regulator is operatively coupledto a single-phase of a three-phase power system to regulate a voltage ofthe single-phase to an in-band area for delivery to a load. Theapparatus includes a means for deriving a digitized voltage samplestream representative of a time-varying measured voltage of thesingle-phase, and a microcontroller operatively coupled to the means forderiving the digitized voltage sample stream. The microcontrollerincludes, among other things, a microprocessor and a memory operativelycoupled to the microprocessor. The microcontroller is programmed to (1)start a countdown time period upon detecting a first excursion of themeasured voltage from the in-band area to an out-of-band area, (2)periodically sample and store the measured voltages as a plurality ofmeasured voltage samples, (3) upon expiration of the countdown timeperiod, calculate a measured percentage time based on the plurality ofmeasured voltage samples stored during the countdown window and (4)compare the measured percentage time to a first threshold percentagetime value. The comparison of the measured percentage time to the firstthreshold percentage time value determinative of whether the tapposition change of the voltage regulator is needed. The measuredpercentage time equals a percentage of time over the countdown timeperiod that the measured voltage is in one of the out-of-band voltageranges, or the percentage of time over the countdown time period thatthe measured voltage is in the in-band voltage range.

In accordance with a further aspect of the invention, an apparatus andmethod are provided for controlling operation of a voltage regulator viaa tap position change where the voltage regulator is operatively coupledto a single-phase of a three-phase power system to regulate a voltage ofthe single-phase to an in-band area for delivery to a load. Theapparatus includes a means for deriving a digitized voltage samplestream representative of a time-varying measured voltage of thesingle-phase, and a microcontroller operatively coupled to the means forderiving the digitized voltage sample stream. The microcontrollerincludes, among other things, a microprocessor and a memory operativelycoupled to the microprocessor. The microcontroller is programmed to (1)start a countdown time period upon detecting a first excursion of themeasured voltage from the in-band area to an out-of-band area, (2)periodically sample and store the measured voltages as a plurality ofmeasured voltage samples, (3) upon expiration of the countdown timeperiod, calculate an averaged measured voltage value based on theplurality of measured voltage samples, and (4) compare the averagedmeasured voltage value to a first threshold voltage value. Thecomparison of the averaged measured voltage value to the first thresholdvoltage value is determinative of whether a tap position change of thevoltage regulator is needed. The averaged measured voltage value isbased on a sum of the magnitudes of the plurality of measured voltagesamples stored during the countdown time period, divided by the numberof the plurality of measured voltage samples.

In accordance with yet a further aspect of the invention, a method isprovided for controlling operation of a voltage regulator via a tapposition change where the voltage regulator is operatively coupled to asingle-phase of a three-phase power system to regulate a voltage of thesingle-phase to an in-band area for delivery to a load. The methodincludes recording a first elapsed time period between detecting a firstexcursion of the measured voltage from the in-band area to anout-of-band area and detecting a first return of the measured voltage tothe in-band area. The first excursion initiates a first countdown timeperiod. The method also includes recording a second elapsed time periodbetween detecting the first return of the measured voltage to thein-band area and a second excursion of the measured voltage from thein-band area to an out-of-band area, comparing the second elapsed timeperiod to a predetermined dip time period, and if the second elapsedtime period is less than the predetermined dip time period and if asecond return of the measured voltage to the in-band area is notdetected, causing the tap position change of the voltage regulator uponexpiration of the first countdown time period. If the second elapsedtime period is more than the predetermined dip time period and if asecond return of the measured voltage to the in-band area is notdetected, the method further includes causing a tap position change ofthe voltage regulator upon expiration of a second countdown time periodinitiated upon the second excursion. The tap position change adjusts themeasured voltage from the out-of-band area to the in-band area.

In accordance with a still further aspect of the invention, a method isprovided for controlling operation of a voltage regulator via a tapposition change where the voltage regulator is operatively coupled to asingle-phase of a three-phase power system to regulate a voltage of thesingle-phase to an in-band area for delivery to a load. The methodincludes periodically sampling and storing a plurality of measuredvoltage samples representative of a plurality of the measured voltagesof the single-phase upon detecting a first excursion of the measuredvoltage from the in-band area to an out-of-band area, and uponexpiration of a countdown time period started upon detecting the firstexcursion, comparing a calculated value to a first threshold percentagetime value to determine whether the tap position change of the voltageregulator is needed. The calculated value is based on a comparison ofthe plurality of measured voltage samples stored during the countdowntime period and the countdown time period. The calculated value may be(A) a measured percentage time equal to a percentage of time over thecountdown time period that the measured voltage is in one of either thein-band area or in one of the out-of-band areas, or (B) an averagedmeasured voltage value equal to a sum of the magnitudes of the pluralityof measured voltage samples divided by the number of the plurality ofmeasured voltage samples. The sum may include all of the measuredvoltage samples or a portion of the measured voltage samples.

It should be understood that the present invention includes a number ofdifferent aspects or features which may have utility alone and/or incombination with other aspects or features. Accordingly, this summary isnot exhaustive identification of each such aspect or feature that is nowor may hereafter be claimed, but represents an overview of certainaspects of the present invention to assist in understanding the moredetailed description that follows. The scope of the invention is notlimited to the specific embodiments described below, but is set forth inthe claims now or hereafter filed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a single line schematic diagram of a power system that may beutilized in a typical wide area.

FIG. 2 is a schematic diagram illustrating a configuration of thevoltage regulator with voltage control device of FIG. 1, according to anembodiment of the invention.

FIG. 3 is a block diagram of an exemplary configuration of the voltagecontrol device of FIG. 2.

FIG. 4 is an exemplary graphic illustrating the center-band area andassociated out-of-band areas that may be used by the voltage controldevice of FIG. 2, according to an embodiment of the invention.

FIG. 5 is a flowchart of a method for providing an adaptive timecharacteristic for use by the voltage control device of FIG. 2,according to an embodiment of the invention.

FIG. 6 is a graphical representation of the adaptive time characteristicof FIG. 5.

FIG. 7 is another flowchart of a method for providing an adaptive timecharacteristic for use by the voltage control device of FIG. 2,according to an embodiment of the invention.

FIGS. 8A, 8B and 8C are a series of flowcharts of a method for providinga percentage time characteristic for use by the voltage control deviceof FIG. 2, according to an embodiment of the invention.

FIG. 9 is a graphical representation of the percentage timecharacteristic of FIG. 8.

FIG. 10 is a flowchart of a method for providing an average voltagecharacteristic for use by the voltage control device of FIG. 2,according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

An apparatus and methods are provided in a voltage control device forcontrolling operation of a single-phase voltage regulator in athree-phase power system. As noted above, one drawback when implementingthe definite time characteristic is that a First countdown timer resetseach time the measured voltage leaves the OOB area and “dips” back intothe in-band area; even if that dip is momentary. According to anembodiment of the invention, an adaptive time characteristic is providedthat permits one dip back into the in-band area during a countdown timeperiod without affecting the countdown time period of the First timer.As a result, the adaptive time characteristic addresses the problem ofmultiple start/stop timing sequences resulting from single large loads(such as arc furnaces) causing the measured voltage to momentarily dipin-band, thereby resetting the First timer and delaying an adjustment toa tap position of the voltage regulator to drive the measured voltageback into the desired in-band area.

As noted above, another drawback of the definite time characteristic isthat when the measured voltage oscillates around a band edge, dipping inand out of the in-band area, the voltage regulator may never operate ormay experience delayed operation due to resetting of the First timer.According to another embodiment of the invention, a percentage timecharacteristic is provided to enable a tap change position uponexpiration of the countdown time period of the First timer under certainconditions. A tap position change is enabled if, after a first excursioninto the OOB area, the percentage of time the measured voltage over thecountdown time period is in the OOB area more than a selected percentageof time, or more than “a first threshold percentage time value”. Thepercentage of time the measured voltage over the countdown time periodis in the OOB area (or in the in-band area, depending on enablement) isherein referred to as “a measured percentage time”. Unlike the adaptivetime characteristic that allows only one dip back into the in-band area,the percentage time characteristic places no restriction on the numberof dips into the in-band area over the countdown time period.

According to yet another embodiment of the invention, an average voltagecharacteristic is provided to enable a tap change position uponexpiration of a countdown time period of the First timer if the averagemeasured voltage value over the countdown time period exceeds athreshold voltage value, after a first excursion into the out-of-bandarea. Unlike the adaptive time characteristic that allows only one dipback into the in-band area, the average voltage characteristic places norestriction on the number of dips into the in-band area.

FIG. 1 is a single line schematic diagram of a power system 10 that maybe utilized in a typical wide area. As illustrated in FIG. 1, the powersystem 10 includes, among other things, three generators 12 a, 12 b and12 c, configured to generate three-phase sinusoidal waveforms such as 12kV sinusoidal waveforms, three step-up power transformers 14 a, 14 b and14 c, configured to increase the generated waveforms to a higher voltagesinusoidal waveforms such as 138 kV sinusoidal waveforms and a number ofcircuit breakers 18. The step-up power transformers 14 a, 14 b, 14 coperate to provide the higher voltage sinusoidal waveforms to a numberof long distance transmission lines such as the transmission lines 20 a,20 b and 20 c. In an embodiment, a first substation 16 may be defined toinclude the two generators 12 a and 12 b, the two step-up powertransformers 14 a and 14 b and associated circuit breakers 18, allinterconnected via a first bus 19. A second substation 35 may be definedto include the generator 12 c, the step-up power transformer 14 c andassociated circuit breakers 18, all interconnected via a second bus 25.At the end of the long distance transmission lines 20 a, 20 b, a thirdsubstation 22 includes two step-down power transformers 24 a and 24 bconfigured to transform the higher voltage sinusoidal waveforms to lowervoltage sinusoidal waveforms (e.g., 15 kV) suitable for distribution viaone or more distribution lines.

As illustrated, the second substation 35 also includes two step-downpower transformers 24 c and 24 d on respective distribution lines 28 and29 to transform the higher voltage sinusoidal waveforms, received viathe second bus 25, to lower voltage sinusoidal waveforms. A (line)voltage regulator 32 is included on the load side of the powertransformer 24 c to provide voltage regulation for the load 30, and avoltage regulator 37, identically configured and operable as the voltageregulator 32, is included on the load side of the power transformer 24 dto provide voltage regulation to the load 34. For example, the voltageregulator 32 may be designed to provide 13 kV±10% for distribution viaan A-phase distribution line 28 to the load 30.

Voltage control devices 100 and 101 are operatively coupled torespective voltage regulators 32, 37, and execute a voltage controlscheme (discussed below), to provide control for their associatedvoltage regulators 32, 37. Although illustrated as a single lineschematic diagram for ease of discussion, it should be noted that eachof the A-, B- and C-phase distribution lines may include a single-phasevoltage regulator such as the voltage regulator 32 and an associatedvoltage control device such as the voltage control device 100.

FIG. 2 is a schematic diagram illustrating a configuration of thevoltage regulator 32 with the voltage control device 100, according toan embodiment of the invention. As noted above, each phase distributionline of the A-, B- and C-phase power system may include its own voltageregulator and voltage control device. For ease of discussion and examplehowever, the voltage regulator 32 and the voltage control device 100 areoperatively coupled to an A-phase distribution line 28.

As was also noted above, because the voltage control device 100 isdesigned to utilize currents and voltages much less than those of adistribution line such as, for example, the A-phase distribution line28, transformers are provided. In the illustrated example, the voltagecontrol device 100 is coupled to the A-phase distribution line 28 viaone current transformer 36 and one voltage transformer 40. The voltagetransformer 40 is used to step-down the power system voltage to asecondary voltage waveform V_(SA) 46 having a magnitude that can bereadily monitored and measured by the voltage control device 100 (e.g.,to step-down the distribution line voltage from 13 kV to 120 V).Similarly the current transformer 36 is utilized to proportionallystep-down the power system line current to a secondary current I_(SA) 44having a magnitude that can be readily monitored and measured by thevoltage control device 100 (e.g., step-down the distribution linecurrent from 200 amps to 0.2 amps). A second voltage transformer 38 mayalso be included for use during a reverse load condition (i.e., agenerator is switched in on the load side). As shown, each of thecurrent transformer 36 and the voltage transformer(s) 40 are included inthe voltage regulator 32, however other arrangements of the voltageregulator 32, the voltage control device 100 and associated transformersare contemplated.

When received by the voltage control device 100, the A-phase secondarycurrent and A-phase-to-ground voltage are filtered, processed andutilized by a microcontroller 130 to calculate phasors havingcorresponding magnitudes and phase angles. The phasors are used by themicrocontroller 130 to determine whether a tap change is needed toadjust the load voltage back into the center-band (e.g., adjust to 120V) FIG. 3 is a block diagram of an exemplary configuration of thevoltage control device 100. During operation of the voltage controldevice 100, the secondary current waveform I_(SA) 44 resulting from thecurrent transformer 36 is further transformed into a correspondingvoltage waveform via a current transformer 104 and a resistor (notseparately illustrated), and filtered via an analog low pass filter 114.The secondary voltage waveform V_(SA) 46 resulting from the voltagetransformer 40 is similarly processed and filtered via another analoglow pass filter 116. An analog-to-digital (A/D) converter 120 thenmultiplexes, samples and digitizes the filtered secondary current andsecondary voltage waveforms to form a corresponding digitized currentand voltage signal 124.

The corresponding digitized current and voltage signal 124 is receivedby a microcontroller 130, where it is digitally filtered via, forexample, Cosine filters to eliminate DC and unwanted frequencycomponents. In an embodiment, the microcontroller 130 includes a CPU, ora microprocessor 132, a program memory 134 (e.g., a Flash EPROM) and aparameter memory 136 (e.g., an EEPROM). As will be appreciated by thoseskilled in the art, other suitable microcontroller configurations (orFPGA configurations) may be utilized. Further, although discussed interms of a microcontroller, it should be noted that the embodimentspresented and claimed herein may be practiced using an FPGA or otherequivalent.

The microprocessor 132, executing a computer program or voltage controllogic scheme (discussed below in connection to FIG. 4), processes (eachof) the digitized current and voltage signal 124 to extract phasorsrepresentative of a corresponding measured secondary voltage V_(SA) 46and current I_(SA) 44, and then performs various calculations using thephasors to determine whether the measured secondary voltage V_(SA) 46 isin either of the first or second OOB areas 154, 156. If such an OOBcondition occurs, the microprocessor 132 issues a tap change command tothe voltage regulator 32 to cause a tap change (i.e., change theeffective turns ratio) to adjust the A-phase-to-ground voltage to thedesired center-band voltage 153, or reference voltage.

As was noted above, voltage regulators generally operate via acomparison of an actual measured secondary voltage V_(SA) 46 to someinternal fixed reference voltage, typically the center-band voltage 153.FIG. 4 is an exemplary graphic 150 illustrating the in-band area 152,including the center-band voltage 153, and associated OOB areas 154, 156that may be used by the voltage control device of 100, according to anembodiment of the invention. Although assigned voltage values fordiscussion purposes, it should be noted that the in-band area 152 andthe first and second OOB areas 154, 156 may include different voltagevalues.

As illustrated, a center-band voltage 153 included within an in-bandarea 152 is selected to be 120 V±2V for a total in-band area width of 4V. As a result, the first OOB area 154 begins at a first in-band/OOBedge 155 at 122V and extends upward beyond 128V, where 128V is themaximum voltage above which tap RAISE commands are suspended by thevoltage control device 100. The second OOB area 156 begins at a secondin-band/OOB edge 157 at 118V and extends downward beyond 109V, where109V is the minimum voltage below which tap LOWER commands are suspendedby the voltage control device 100. A deadband area 158 is establishedbetween 128V and a runback voltage of 130V in order to effect fastvoltage correction because of an extreme voltage condition. When themeasured secondary voltage V_(SA) 46 is equal to or above the runbackvoltage, the voltage control device issues a tap LOWER command withoutany time delay.

As was also noted above, when using a well-known definite timecharacteristic, under conditions of the measured voltage V_(SA) 46oscillating around either of the first or second in-band/OOB edges 155,157, the voltage control device 100 may not issue a needed tap positionchange command to the voltage regulator 32 due to the repeated Firsttimer resets. While reducing voltage regulator maintenance time, theabsence of needed tap position changes results in inefficient voltageregulation.

In accordance with an embodiment of the invention, provided is anadaptive time characteristic where, rather than resetting a First timerto its countdown time period T_(S1) (see FIG. 6) upon a first return ofthe measured voltage V_(SA) 46 into the in-band area 152 following afirst excursion into the first OOB area 154, or a first excursion cycle,the microcontroller 130 adjusts the countdown time period T_(S1) of theFirst timer to a new, or adjusted countdown time period T_(SA) undercertain circumstances. In general, the adjusted countdown time periodT_(SA) reflects a “dip time” T_(D) back into the in-band area 152 andthe elapsed time of T_(E) the first excursion cycle. More specifically,upon detecting a second excursion into the first OOB area 154, and themicrocontroller 130 causes the First timer to subtract from itscountdown time period T_(S1) both (1) the time elapsed T_(E) between thefirst return into the in-band area 152 and the second excursion into thefirst OOB area 154 (“dip time T_(D)”), and (2) the time elapsed T_(E)due to the first excursion cycle, if that dip time T_(D) is less than apredetermined dip time period. Although discussed below in terms ofexcursions to and from the first OOB area 154, it should be noted thatthe adaptive time characteristic discussed herein is equally applicableto excursions to and from the second OOB area 156.

If the dip time T_(D) of the measured voltage V_(SA) 46 is less than thepredetermined dip time period and if the measured voltage V_(SA) 46 doesreturn to the OOB area 154 a second time, the microcontroller 130effectively causes the First timer to continue its countdown time periodT_(S1) as if the first dip had not occurred. This allows for a randomvoltage dip into the in-band area 152 that may have occurred due to anoccasional single large load such as an arc furnace coming on-line andcausing a momentary voltage dip, without delaying a needed tap positionchange. If the dip time T_(D) between the first return into the in-bandarea 152 and the second excursion into the first OOB area 154 is greaterthan the predetermined dip time period however, the microcontroller 130causes the First timer to again begin its countdown time period upon thesecond excursion into the first OOB area 154.

FIG. 5 is a flowchart of a method 200 for providing an adaptive timecharacteristic for use by the voltage control device 100, according toan embodiment of the invention. Referring to FIG. 5, the method 200begins when the microcontroller 130 starts a countdown time periodT_(S1) of the First timer upon detecting a first excursion of themeasured voltage V_(SA) 46 into the first OOB area 154 from the in-bandarea 152 (step 202). Upon detecting a first return of the measuredvoltage V_(SA) 46 to the in-band area 154, the microcontroller 130 (a)records an elapsed time T_(E) based on a time elapsed between the firstexcursion of the measured voltage V_(SA) 46 into the first OOB area 154and its subsequent first return into the in-band area 152, or a timeelapsed during the first excursion cycle, (b) resets the First time toits countdown time period T_(S1), and (c) starts a Second timer tomeasure a dip time T_(D) between the first return of the measuredvoltage V_(SA) 46 into the in-band area 152 and a second excursion intothe first OOB area 154 (step 204). Upon detecting a second excursion ofthe measured voltage from the in-band area 152 to the first OOB area154, the microcontroller 130 records the dip time T_(D) (or second timeelapsed time) of the Second timer, and compares the dip time T_(D) to apredetermined dip time period (step 206).

If the dip time T_(D) is determined to be less than the predetermineddip time period (step 207), the microcontroller 130 starts an adjustedcountdown time period T_(SA) of the First timer, where the adjustedcountdown time period T_(SA) is calculated by subtracting the elapsedtime T_(E) and the dip time T_(D) from the adjusted countdown timeperiod, or T_(SA)=T_(S1)−(T_(E)+T_(D)) (step 208). If the dip time T_(D)is determined to be more than the predetermined dip time period (step207), the microcontroller 130 starts the countdown time period T_(S1) ofthe First timer (step 210). If a second return of the measured voltage46 to the in-band area 152 does not occur (step 209), then uponexpiration of the adjusted countdown time period T_(SA), themicrocontroller 130 causes a tap position change of the voltageregulator (step 212). The tap position change adjusts the measuredvoltage V_(SA) 46 from the first OOB area 154 to the in-band area 152.If a second return of the measured voltage V_(SA) 46 to the in-band area152 does occur, the microcontroller 130 causes the First timer to resetto its countdown time period T_(S1). The First timer does not begin tocountdown however until a subsequent excursion into the first OOB area154 is detected.

For example, FIG. 6 is a graphical representation 220 of the adaptivetime characteristic, according to an embodiment of the invention. Thegraphical representation 220 is useful for describing a numericalexample of the adaptive time characteristic. For ease of discussion, itwill be assumed that the First timer has a countdown time period T_(S1)of 120 seconds, and that the predetermined dip time period is 10seconds.

As illustrated in FIG. 6, a First timer begins its 120 second countdowntime period T_(S1) upon a first excursion of the measured voltage V_(SA)46 into the first OOB area 154 at t=0. Four seconds later, the measuredvoltage V_(SA) 46 returns to the in-band area 152, yielding an elapsedtime T_(E)=4. In addition, the First timer resets to its 120 secondcountdown time period T_(S1) again, and the Second timer begins tomeasure the dip time T_(D). Five seconds later, a second excursion intothe first OOB area 154 occurs at t=9, yielding a measured dip time T_(D)of 5 seconds (via the Second timer). The First timer begins itscountdown again. Because the 5 second dip time T_(D) is less than thepredetermined dip time period of 10 seconds, the First timer uses anadjusted time setting T_(SA) of 111 seconds (i.e., (120 seconds)−(4seconds+5 seconds)=111 seconds), effectively allowing the First timer to“ride through” the 5 second voltage dip into the in-band area 152. Uponexpiration of the adjusted countdown time period T_(SA) of the Firsttimer, the microcontroller 130 causes a tap position change at t=120seconds. Alternatively, in a case where the measured dip time T_(D) isgreater than the predetermined dip time period, for example, 11 seconds,the microcontroller 130 causes a tap position change at t=120 secondsfrom the second start of the First timer, or t=131 seconds from thefirst start of the First timer.

The adaptive time characteristic may also be provided without stoppingthe countdown time period of the First timer upon each return of themeasured voltage V_(SA) 46 to the in-band area 152 following anexcursion of the measured voltage V_(SA) 46 into the first OOB area 154.For example, FIG. 7 is a flowchart of another method 230 for providingan adaptive time characteristic for use by the voltage control device100, according to an embodiment of the invention. Referring to FIG. 7,the method 230 begins when the microcontroller 130 starts a countdowntime period T_(S1) of the First timer upon detecting a first excursionof the measured voltage V_(SA) 46 into the first OOB area 154 (step232). Upon detecting a first return of the measured voltage V_(SA) 46 tothe in-band area 154, the microcontroller 130 continues the First timercountdown and starts the Second timer to measure a dip time T_(D)between the first return into the in-band area 152 and a secondexcursion into the first OOB area 154 (step 234).

Upon detecting a second excursion of the measured voltage V_(SA) 46 fromthe in-band area 152 to the first OOB area 154, the microcontroller 130records the dip time T_(D) of the Second timer, and compares the diptime T_(D) to a predetermined dip time period (step 236). If the diptime T_(D) is determined to be less than the predetermined dip timeperiod (step 237), the microcontroller 130 ignores the dip and continuesthe First timer countdown, as if the dip never occurred. If a secondreturn of the measured voltage V_(SA) 46 to the in-band area 152 is notdetected prior to expiration of the countdown time period, themicrocontroller 130 causes a tap position change of the voltageregulator upon expiration of the countdown time period (step 238). Thetap position change adjusts the measured voltage V_(SA) 46 from thefirst OOB area 154 to the in-band area 152. If the dip time T_(D) isdetermined to be more than the predetermined dip time period (step 237),the microcontroller 130 causes the First timer to reset and begin thecountdown time period (step 240). If a second return of the measuredvoltage V_(SA) 46 to the in-band area 152 does occur, themicrocontroller 130 causes the First timer to reset to its countdowntime period. The First timer however, does not begin its countdown timeperiod until the microcontroller 130 detects a subsequent excursion intothe first OOB area 154 is detected.

In accordance with yet another embodiment of the invention, provided isa percentage time characteristic where, rather than terminating thecountdown time period T_(S1) of the First timer at the end of eachexcursion cycle in the first OOB area 154, the microcontroller 130causes the countdown to continue. Further, based on a plurality ofmeasured voltage samples taken over a selected time period (e.g., afirst countdown time period), the microcontroller 130 calculates apercentage of time of the countdown time period T_(S1) for which themeasured voltage V_(SA) 46 is in the first OOB area 154, or calculatesthe measured percentage time. The measured percentage time is thencompared to a first threshold percentage time value to determine whethera tap position change is needed at the end of a countdown time period.In some cases, the measured percentage time is also compared to a secondthreshold percentage time value.

Although described in terms of the first OOB area 154 for ease ofdiscussion, it should be noted that the percentage time characteristicis applicable to a percentage of time during the countdown time periodthat the measured voltage V_(SA) 46 is in the second OOB area 156 belowthe in-band area 152, or the percentage of time during the countdowntime period that the measured voltage V_(SA) 46 is in the in-band area152. Further, although the first and second threshold percentage timevalues are selected as a percentage of time that the measured voltageV_(SA) 46 is in the first OOB area 154, the first and second thresholdpercentage time values may be based on other percentages such as apercentage of time the measured voltage V_(SA) 46 is in the in-band area152 or in the second OOB area 156, etc. In addition, althoughexemplified using first and second threshold percentage time values, itis contemplated that only one threshold percentage time value may beused to determine whether a tap position change is required.

FIGS. 8A, 8B and 8C are a series of flowcharts of a method 250 forproviding a percentage time characteristic for use by the voltagecontrol device 100, according to an embodiment of the invention. Thecountdown time period T_(S1) of the First timer is associated with acountdown window, and begins with a first excursion of the measuredvoltage V_(SA) 46 into the OOB area 154. Unlike the definite timecharacteristic however, a first return into the in-band area 152 doesnot cause the countdown time period T_(S1) to terminate. Rather, upondetecting the first excursion of the measured voltage V_(SA) 46 into theOOB area 154, the microcontroller 130 causes a first countdown timeperiod T_(S1) to begin (step 252).

During the first countdown time period, the microcontroller 130periodically samples (e.g., 2 samples per second) and stores themeasured voltages V_(SA) 46 as a plurality of measured voltage samples(step 254). Upon expiration of the countdown time period (i.e., at theend of the first countdown window), the microcontroller 130 calculates ameasured percentage time based on the plurality of measured voltagesamples (step 256). The measured percentage time is the percentage oftime over the countdown time period T_(S1) that the measured voltageV_(SA) 46 is in the first OOB area 154. The measured percentage time mayalso be the percentage of time over the countdown time period T_(S1)that the measured voltage V_(SA) 46 (magnitude) is in the second OOBarea 156. Similarly, the measured percentage time may also be thepercentage of time over the countdown time period T_(S1) that themeasured voltage V_(SA) 46 is in the in-band area 152.

Next the microcontroller 130 compares the measured percentage time to afirst threshold percentage time value (step 258). The comparison isdeterminative of whether a tap position change is needed. For a firstthreshold percentage time value based on the first OOB area 154 (step259), if the measured percentage time is more than the first thresholdpercentage time value, indicating that the percentage of time themeasured voltage V_(SA) 46 is in the first OOB area 154 exceeds what isallowable per the first threshold percentage time value (step 261), themicrocontroller 130 immediately causes a tap position change of thevoltage regulator to adjust the measured voltage V_(SA) 46 from thefirst OOB area 154 to the in-band area (step 260). For example, upondetecting expiration of the First timer, the microcontroller 130compares a measured percentage time of 83% in the first OOB area 154 toa first threshold percentage time value of 80%, and because the measuredpercentage time exceeds the first threshold percentage time value, themicrocontroller 130 causes a tap position change.

Referring to FIGS. 8A and 8B, for a first threshold percentage timevalue based on the first OOB area 154, if the measured percentage timeis less than the first threshold percentage time value but is greaterthan a second threshold percentage time value (step 265), themicrocontroller 130 initiates a moving window (step 264), periodicallysamples and stores a plurality of new measured voltage samples of themoving window (step 266) and continuously calculates the measuredpercentage time as the percentage of time over a “moving countdown timeperiod” that the measured voltage V_(SA) 46 is in the first OOB area 154(step 268). Calculation of the measured percentage time continues untilthe measured percentage time either (1) exceeds the first thresholdpercentage time value (step 267) at which time the microcontroller 130causes a tap position change to bring the measured voltage V_(SA) 46back into the In-band area 152 (step 260), or (2) drops below the secondthreshold percentage time value (step 269) at which time themicrocontroller 130 resets the First timer (step 257). After resettingthe First timer, upon a next excursion of the measured voltage V_(SA) 46into the first OOB area 154, the microcontroller 130 causes thecountdown time period T_(S1) of the First timer to again begin (step252). Similarly, for cases where the measured percentage time is thepercentage of time the measured voltage V_(SA) 46 is in the second OOBarea 156, upon a next excursion of the measured voltage V_(SA) 46 intothe second OOB area 156, the microcontroller 130 causes the countdowntime period T_(S1) of the First timer to again begin.

Referring again to FIG. 8A, for a first threshold percentage time valuebased on the in-band area 152 (step 262), if the measured percentagetime is less than the first threshold percentage time value (step 263),indicating that the percentage of time the measured voltage V_(SA) 46 isin the in-band area 156 is less than what is required per the firstthreshold percentage time value, the microcontroller 130 immediatelycauses a tap position change of the voltage regulator to adjust themeasured voltage V_(SA) 46 from the out-of-band area to the in-band area(step 260). For example, upon detecting expiration of the First timer,the microcontroller 130 compares a measured percentage time of 13% inthe in-band area 152 to a first threshold percentage time value of 20%,and because the measured percentage time is less the first thresholdpercentage time value, the microcontroller 130 causes a tap positionchange.

Referring to FIG. 8C, for a first threshold percentage time value basedon the in-band area 152, if the measured percentage time is more thanthe first threshold percentage time value (e.g., 20%) but is less than asecond threshold percentage time value (e.g., 25%), the microcontroller130 initiates a moving window (step 264), periodically samples andstores the measured voltages (step 266) and continuously calculates themeasured percentage time as the percentage of time over the movingcountdown time period that the measured voltage V_(SA) 46 is in thein-band area 152 (step 268). Calculation of the measured percentage timecontinues until the measured percentage time either (1) drops below thefirst threshold percentage time value (step 271) at which time themicrocontroller will cause a tap position change to bring the measuredvoltage V_(SA) 46 back into the In-band area 152 (step 260), or (2)exceeds the second threshold percentage time value (step 273) at whichtime the microcontroller resets the First timer (step 257). After themeasured percentage time either drops below the first thresholdpercentage time value or exceeds the second threshold percentage timevalue, upon a next excursion of the measured voltage V_(SA) 46 into thefirst OOB area 154 (or the second OOB area 156), the First timer againbegins its countdown time period T_(S1) (step 252).

In an embodiment, the moving window includes present and past measuredvoltage samples over a time period equivalent to the countdown timeperiod of the First timer, or the moving countdown time period. It iscontemplated however that the moving window may include present and pastmeasured voltage samples over a time period different from the countdowntime period of the First timer.

For example, if at the expiration of the First timer at 120 seconds, themicrocontroller 130 determines that the measured percentage time is lessthan the first threshold percentage time value but is greater than firstthreshold percentage time value, a moving window that includes measuredvoltage samples generated during a 120 second period, begins. Thus at 2samples per second and at 122 seconds, the moving window includes 240measured voltage samples generated between 2 seconds following the startof the First timer countdown time period and two seconds followingexpiration of the First timer countdown time period. Similarly, at 123seconds, the moving window includes 240 measured voltage samplesgenerated between 3 seconds following the start of the First timercountdown time period and 3 seconds following expiration of the Firsttimer countdown time period.

As noted above, for first the microcontroller 130 continues to calculatethe measured percentage time until the measured percentage time either(1) exceeds the first threshold percentage time value or (2) drops belowthe second threshold percentage time value. For example, if the firstthreshold percentage time value is 80% and the second thresholdpercentage time value is 60%, and the measured percentage time is 70% atthe end of the countdown time period T_(S1), the microcontroller 130initiates a moving window and then begins to continuously calculate themeasured percentage time as the percentage of time over the movingcountdown time period that the measured voltage V_(SA) 46 is in thefirst OOB area 154 (or in the second OOB area 156).

If at, for example, 152 seconds, the measured percentage time is 80.01%thereby exceeding the first threshold percentage time value of 80%, themicrocontroller 130 causes a tap position change to adjust the measuredvoltage V_(SA) 46 from the first OOB area 154 to the in-band area 152.The measured percentage time of 80.01% indicates that 80.01% of the timeover the 120 second moving window, the measured voltage samples were inthe first OOB area 154 (or exceeded the in-band/OOB edge 155) and in thein-band area 152 only 19.99% of the time. On the other hand, if at 152seconds the measured percentage time is 59.99%, below the secondthreshold percentage time value of 60%, the First timer is reset to itscountdown time period T_(S1) and no tap position change occurs. Themeasured percentage time of 59.99% indicates that 59.99% of the timeover the 120 second moving window, the measured voltage samples were inthe first OOB area 154 (or exceeded the in-band/OOB edge 155), and inthe in-band area 152 40.01% of the time. Following the First timerreset, upon detecting a next excursion of the measured voltage V_(SA) 46into the first OOB area 154, the First timer begins its countdown timeperiod again.

In addition to periodically sampling and storing the measured voltagesV_(SA) 46 directly, the microcontroller 130 may further assign a binaryvalue to the each measured voltage sample and then store the binaryvalues. For example, the microcontroller 130 may assign and store abinary 1 for each measured voltage sample in the first OBB area 154 (oreach measured voltage sample in the second OOB area 156), and may assigna binary 0 value for each measured voltage sample in the in-band area152. Either the percentage of the binary 1 values or the percentage ofthe binary 0 values may then be compared to the first and secondthreshold percentage time values to determine whether a tap positionchange is needed.

For example, FIG. 9 is graphical representation 275 of the percentagetime characteristic, according to an embodiment of the invention. Thegraphical representation 275 is useful for describing a numericalexample of the percentage time characteristic. For ease of discussion,it will be assumed that the First timer has a countdown time periodT_(S1) of 120 seconds, and that a first threshold percentage time valueis 80% and a second threshold percentage time value is 60%.

Implementation of the percentage time characteristic may be accomplishedin one of any number of ways. For example, the voltage may be sampledperiodically to form measured voltage samples that are stored in thememory 134 and then used to calculate the measured percentage time atthe end of the fixed countdown window. The voltage may also be sampledperiodically to form measured voltage samples that are used tocontinuously calculate the measured percentage time. The periodicsampling rate may be a periodic number of times per power cycle, or maybe a periodic number of times per second, etc. For ease of illustration,it will be assumed that the periodic sampling rate for the measuredvoltage V 46 is equal to one measured voltage sample per second. It willalso be assumed that a measured voltage sample above 122 V (or ameasured voltage sample below 118 V) yields a binary 1 value, while ameasured voltage sample equal to or between 122 V and 118 V, yields abinary 0 value. Further, the first threshold percentage time value isassumed to be 80%; that is, if 80% or more of the 120 binary values arebinary 1 values (more than 96), the microcontroller 130 will cause a tapposition change. The second threshold percentage time value is assumedto be 60%; that is, 60% or more of the 120 binary values are binary 1values (more than 72).

As illustrated in FIG. 9, a First timer begins a first countdown windowhaving a 120 second countdown time period T_(S1) upon a first excursionof the measured voltage V_(SA) 46 into the first OOB area 154 at t=0.Four seconds later, the measured voltage V_(SA) 46 returns to thein-band area 152, marking the end of the first excursion cycle andyielding four binary 1 values and an elapsed time of 4 seconds. Elevenseconds later, the measured voltage V_(SA) 46 enters the first OOB area154 for a second time, marking the beginning of the second excursioncycle and yielding nine binary 0 values and an elapsed time of 15seconds. Two seconds later, the measured voltage V_(SA) 46 returns tothe in-band area 152, marking the end of the second excursion cycle andyielding two binary 1 values and a total elapsed time of 17 seconds. Twoseconds later, the measured voltage V_(SA) 46 enters the first OOB area154 for a third time, marking the beginning of the third excursion cycleand yielding two binary 0 values and an elapsed time of 19 seconds. Themeasured voltage V_(SA) 46 remains in the first OOB area 154 for 3seconds, returning to the in-band area 152 at 22 seconds, completing thethird excursion cycle and yielding three binary 1 values. After onesecond and one binary 0 value, the measured voltage V_(SA) 46 thenreturns to the first OOB area 154 for a fourth time at 23 seconds andremains there. At the end of the 120 second countdown window, the sum ofthe binary 1 values equals the first three excursion cycles of 9 binary1 values plus the fourth excursion into the first OOB area, yielding 97binary 1 values. Therefore, at 120 seconds, the microcontroller 130causes a tap position change to lower the measured voltage V_(SA) 46because 106 binary 1 values yields a measured voltage percentage of 88%;greater than the 80% first threshold percentage time value.

Referring again to FIG. 8A, because the measured voltage percentage isgreater than the first threshold percentage time value, a moving windowis not initiated by the microcontroller 130. Instead, themicrocontroller 130 causes a tap position change to adjust the measuredvoltage V_(SA) 46 from the first OOB area 154 to the in-band area 152,and causes the First timer to reset back to its countdown time period.

According to a further embodiment of the invention, an average voltagecharacteristic is provided to enable a tap change position uponexpiration of a countdown time period of the First timer if, after afirst excursion into the out-of-band area, the averaged measured voltageV_(SA) 46 over the countdown time period exceeds a first thresholdvoltage value. As with the percentage time characteristic describedabove, rather than terminating the countdown time period T_(S1) of theFirst timer at the end of each excursion cycle in the first OOB area154, the microcontroller 130 calculates an average voltage over thecountdown time period T_(S1) to form an averaged measured voltage value.In general, the averaged measured voltage value is then compared to thefirst threshold voltage value, such as the voltage value of the firstin-band/OOB edge 155, at the end of the countdown time period T_(S1) todetermine whether a tap position change is needed. In some case, theaveraged measured voltage value is also compared to a second thresholdvoltage value.

Although described in terms of the first OOB area 154, it should benoted that the average voltage characteristic is applicable to anaveraged voltage in the second OOB area 156 below the in-band area 152,or to an averaged voltage in the in-band area 152. Further, althoughexemplified using first and second threshold voltage values, it iscontemplated that only one threshold voltage value may be used todetermine whether a tap position change is required.

FIG. 10 is a flowchart of a method 300 for providing an average voltagecharacteristic for use by the voltage control device 100, according toan embodiment of the invention. Referring to FIG. 10, a countdown timeperiod T_(S1) is associated with a First timer, and begins with a firstexcursion of the measured voltage V_(SA) 46 into the OOB area 154.Unlike the well-known definite time characteristic, however, a firstreturn of the measured voltage V_(SA) 46 into the in-band area 152 doesnot cause the countdown time period T_(S1) to terminate. Rather, upondetecting a first excursion of the measured voltage V_(SA) 46 into theOOB area 154 from the in-band area 152, the microcontroller 130 causes afirst countdown time period T_(S1) to begin (step 302). Themicrocontroller 130 periodically samples (e.g., four samples per powersystem cycle) and stores selected measured voltages (e.g., two samplesper second) as a plurality of measured voltage samples representative ofmeasured voltages V_(SA) 46 of the single-phase (step 304).

In an example, the microcontroller 130 may assign a value to each of thesampled measured voltages based on an associated phasor magnitude where,for example, a measured voltage sample having a magnitude of 123 V isassigned a value of 123 and a measured voltage sample having a magnitude119 V is assigned a value of 119, and so on. In this case, the assignedvalue is equal to the magnitude of the sample measured voltage howeverother schemes to represent the measured voltage samples may be utilized.

Referring again to FIG. 10, upon expiration of the countdown time period(i.e., at the end of the first countdown period), the microcontroller130 calculates the averaged measured voltage value based on theplurality of measured voltage samples (step 306), and compares theaveraged measured voltage value to a first threshold voltage value (step308). Calculation of the averaged measured voltage value may include useof all of the measured voltage samples or a portion of the measuredvoltage samples. The comparison is determinative of whether a tapposition change is needed. The averaged measured voltage value is basedon a sum of the magnitudes of the plurality of measured voltage samplesstored during the countdown time period, divided by the number of theplurality of measured voltage samples used. For example, if analogvalues are assigned to each measured voltage samples based on the samplemagnitude as described above, an average of the sample magnitudes (sumof the sample magnitudes divided by the number of samples).

Although the first threshold voltage value is preferably the firstin-band/OOB edge 155 (e.g., 122 V) between the first OOB area 154 andthe in-band area 152, another voltage value may be used. If averagedmeasured voltage value is greater than the first threshold voltage value(step 309), the microcontroller 130 causes a tap position change, eitherimmediately or after a predetermined interval (step 312). If averagedmeasured voltage value is not greater than the first threshold voltagevalue (step 309), the microcontroller 130 compares the averaged measuredvoltage value (step 310) to a second threshold voltage value. Althoughthe second threshold voltage value is preferably the in-band/OOB edge157 (e.g., 118 V) between the in-band area 152 and the second OOB area156, another voltage value may be used.

If averaged measured voltage value is less than the second thresholdvoltage value (step 311), the microcontroller 130 causes a tap positionchange, either immediately or after a predetermined interval (step 312).If averaged measured voltage value is not less than the first thresholdvoltage value (step 309), the microcontroller 130 resets the Firsttimer. The countdown time period T_(S1) does not begin again untilanother excursion of the measured voltage V_(SA) 46 into either thefirst or the second OOB area 154, 156 occurs.

As may be apparent from the above discussion, implementation of theapparatus and method disclosed herein enables improved voltage regulatorcontrol, especially for those cases where an occasional momentary loaddraw occurs or where the measured voltage V_(SA) 46 oscillates aroundthe in-band/OOB edge.

While this invention has been described with reference to certainillustrative aspects, it will be understood that this description shallnot be construed in a limiting sense. Rather, various changes andmodifications can be made to the illustrative embodiments withoutdeparting from the true spirit, central characteristics and scope of theinvention, including those combinations of features that areindividually disclosed or claimed herein. Furthermore, it will beappreciated that any such changes and modifications will be recognizedby those skilled in the art as an equivalent to one or more elements ofthe following claims, and shall be covered by such claims to the fullestextent permitted by law.

1.-21. (canceled)
 22. An apparatus for controlling operation of avoltage regulator via a tap position change, the voltage regulatoroperatively coupled to a single-phase of a three-phase power system toregulate a measured voltage of the single-phase to an in-band area fordelivery to a load, the apparatus comprising: a means for deriving adigitized voltage stream representative of the measured voltage of thesingle-phase; and a microcontroller operatively coupled to the means forderiving the digitized voltage stream, the microcontroller having amicroprocessor and a memory operatively coupled to the microprocessor,the microcontroller being programmed to: start a countdown time periodupon detecting a first excursion of the measured voltage from thein-band area to an out-of-band area, periodically sample and store thedigitized voltage stream as a plurality of measured voltage samples,upon expiration of the countdown time period, calculate an averagedmeasured voltage value based on the plurality of measured voltagesamples, and compare the averaged measured voltage value to a firstthreshold voltage value, the comparison of the averaged measured voltagevalue to the first threshold voltage value determinative of whether thetap position change of the voltage regulator is needed.
 23. Theapparatus of claim 22, wherein the microcontroller is further programmedto cause the tap position change if the averaged measured voltage valueis greater than the first threshold voltage value, the tap positionchange adjusting the measured voltage from the out-of-band area to thein-band area.
 24. The apparatus of claim 22, wherein the microcontrolleris further programmed to: compare the averaged measured voltage value toa second threshold voltage value if the averaged measured voltage valueis not greater than the first threshold voltage value; and if theaveraged measured voltage value is less than the second thresholdvoltage value, cause the tap position change to adjust the measuredvoltage from the out-of-band area to the in-band area.
 25. The apparatusof claim 24, wherein the microcontroller is further programmed to causethe countdown time period to reset if the averaged measured voltagevalue is greater than the second threshold voltage value and less thanthe first threshold value.
 26. The apparatus of claim 24, wherein firstthreshold voltage value is equal to an upper voltage of the in-band areaand wherein the second threshold voltage value is equal to a lowervoltage of the in-band area.
 27. The apparatus of claim 22, wherein thein-band area is adjustable between a first voltage value and a secondvoltage value, and wherein the out-of-band area is above the firstvoltage value.
 28. The apparatus of claim 22, wherein the in-band areais adjustable between a first voltage value and a second voltage value,and wherein the out-of-band area is below the second voltage value. 29.The apparatus of claim 22, wherein the averaged measured voltage valueis equal to a sum of the magnitudes of the plurality of measured voltagesamples divided by the number of the plurality of measured voltagesamples. 30-48. (canceled)
 49. A method for controlling operation of avoltage regulator via a tap position change, the voltage regulatoroperatively coupled to a single-phase of a three-phase power system toregulate a measured voltage of the single-phase to an in-band area fordelivery to a load, the method comprising: deriving a digitized voltagestream representative of the measured voltage of the single-phase;starting a countdown time period upon detecting a first excursion of themeasured voltage from the in-band area to an out-of-band area;periodically sampling and storing a plurality of the measured voltagesas a plurality of measured voltage samples; upon expiration of thecountdown time period, calculating an averaged measured voltage valuebased on the plurality of measured voltage samples; and comparing theaveraged measured voltage value to a first threshold voltage value, thecomparison of the averaged measured voltage value to the first thresholdvoltage value determinative of whether the tap position change of thevoltage regulator is needed.
 50. The method of claim 49, wherein themicrocontroller is further programmed to cause the tap position changeif the averaged measured voltage value is greater than the firstthreshold voltage value, the tap position change adjusting the measuredvoltage from the out-of-band area to the in-band area.
 51. The method ofclaim 49, wherein the microcontroller is further programmed to: comparethe averaged measured voltage value to a second threshold voltage valueif the averaged measured voltage value is not greater than the firstthreshold voltage value; and if the averaged measured voltage value isless than the second threshold voltage value, cause the tap positionchange to adjust the measured voltage from the out-of-band area to thein-band area.
 52. The method of claim 51, wherein the microcontroller isfurther programmed to cause resetting of the countdown time period ifthe averaged measured voltage value is greater than the second thresholdvoltage value and less than the first threshold value.
 53. The method ofclaim 51, wherein first threshold voltage value is equal to an uppervoltage of the in-band area and wherein the second threshold voltagevalue is equal to a lower voltage of the in-band area.
 54. The method ofclaim 51, wherein first threshold voltage value is equal to a lowervoltage of the out-of-band area above the in-band area and wherein thesecond threshold voltage value is equal to a higher voltage of theout-of-band area below the in-band area.
 55. The method of claim 49,wherein the in-band area is adjustable between a first voltage value anda second voltage value, and wherein the out-of-band area is above thefirst voltage value.
 56. The method of claim 49, wherein the in-bandarea is adjustable between a first voltage value and a second voltagevalue, and wherein the out-of-band area is below the second voltagevalue.
 57. The method of claim 49, wherein the averaged measured voltagevalue is equal to a sum of the magnitudes of the plurality of measuredvoltage samples stored during the countdown time period, divided by thenumber of the plurality of measured voltage samples. 58-80. (canceled)